This invention relates to lithographic printing plates and to water-developable, positive acting lithographic printing plates.
Positive acting water developable lithographic printing plates having a photopolymerizable light-sensitive layer are known in the art. Such plates are described, for example, in U.S. Pat. Nos. 4,147,549 and 4,198,236. More particularly, these plates comprise a support, a photosensitive layer or stratum and may have a strippable protective cover layer. The photosensitive layer contains an oleophilic polymer binder which is substantially insoluble in the water developer, a water soluble, ethylenically unsaturated monomer capable of undergoing photoinitiated addition polymerization to form a hydrophilic polymer which is substantially insoluble in the water developer, and an effective amount of a polymerization initiator activatable by actinic radiation and inactive thermally below 85.degree. C. To prepare a positive-working planographic printing plate useful in offset lithography the element is exposed to actinic radiation through a process positive transparency, the protective cover layer is stripped away and the surface of the exposed stratum is washed with water which leaches monomer from the unexposed areas of the stratum. The resulting stratum contains a positive image made up from an oleophilic ink receptive surface from which the monomer has been leached and a complementary, hydrophilic surface comprised of the hydrophilic polymer formed. U.S. Pat. Nos. 3,867,147, 3,085,008 and 3,419,394 relate to lithographic plates comprising an oleophilic layer overcoated with a water-soluble hydrophilic material mixed with a diazo compound, said diazo compound being light sensitive and decomposable so as to effect crosslinking in the hydrophilic material. Upon curing, the hydrophilic polymer becomes water-insoluble but remains hydrophilic.
Although the above-mentioned patents are several years old, as yet, no positive acting water-developable lithographic printing plate is commercially available because wet strength, scratch resistance, and solvent resistance are features for which problems still exist in the current art. Moreover, the state of the art requires gumming or similar types of post treatment or special fountains in order to maintain hydrophilicity of plates after development.
Water developable positive acting lithographic printing plates which employ diazo oxide resin chemistry systems are not favored because of severe light sensitivity problems after development. Because of the nature of the photosolubilization mechanism a water-developable plate coating exposed to actinic radiation after development, during mounting, or running on the press will become soluble in the water based press fountain and will dissolve away. Even if a developed plate could be rigorously protected from light, it is doubtful whether a conventional resin system, when combined with diazo oxide, could provide the water-solubility differential required for clean development and yet be durable when subjected to water on the printing press.
A significant amount of lithographic printing is done from aluminum plates. The aluminum plates are generally grained or roughened prior to coating or processing. This can be done in a flat, circularly oscillating tub by means of steel "marbles", abrasive grit, and water, but it can also be done by dry grit blasting, or brush graining using nylon or steel brushes and abrasive. Grain depth usually varies from 0.5 to 2.5 micrometers. The plate grain provides anchorage for the coating and the ink and recesses that help the surface carry moisture.
It would be advantageous to employ aluminum plates which require a minimum of graining or roughening prior to being coated. It would be even more advantageous to be able to use a plate material made of a synthetic polymeric material, such as polyester. However, adhesion to synthetic polymeric materials by photosensitive layers is a problem that limits durability, i.e. run length, of lithographic printing plates made of polymeric materials.